A220262 Number of even semiprimes < 10^n. Number of terms of A100484 < 10^n.

0, 3, 15, 95, 669, 5133, 41538, 348513, 3001134, 26355867, 234954223, 2119654578, 19308136142, 177291661649, 1638923764567, 15237833654620, 142377417196364, 1336094767763971, 12585956566571620, 118959989688273472, 1127779923790184543, 10720710117789005897

Offset: 0

Author

Robert G. Wilson v, Dec 08 2012

Keywords

• nonn

Comments

All such semiprimes have the form 2*p, where p is prime. - T. D. Noe, Dec 09 2012

Links

• Kim Walisch, Fast C++ prime counting function implementation.

Crossrefs

Cf. A066265, A085770, A100484.

Programs

Mathematica

Table[PrimePi[10^n/2], {n, 0, 14}]

PARI

a(n)=primepi(10^n\2) \\ Charles R Greathouse IV, Sep 08 2015

Python

from sympy import primepi
def A220262(n): return primepi(10**n>>1) # Chai Wah Wu, Oct 17 2024

Formula

a(n) = A066265(n) - A085770(n) for n > 1.

Extensions

a(15)-a(20) from Hugo Pfoertner, Oct 14 2017

a(21) from Jinyuan Wang, Jul 30 2021